Atomistic mechanisms for catalytic transformations of NO to NH₃, N₂O, and N₂ by Pd

Abstract

The industrial pollutant NO is a potential threat to the environment and to human health. Thus, selective catalytic reduction of NO into harmless N, NH₃, and/or N₂O gas is of great interest. Among many catalysts, metal Pd has been demonstrated to be most efficient for selectivity of reducing NO to N₂. However, the reduction mechanism of NO on Pd, especially the route of N−N bond formation, remains unclear, impeding the development of new, improved catalysts. We report here the elementary reaction steps in the reaction pathway of reducing NO to NH₃, N₂O, and N₂, based on density functional theory (DFT)-based quantum mechanics calculations. We show that the formation of N₂O proceeds through an Eley-Rideal (E−R) reaction pathway that couples one adsorbed NO* with one non−adsorbed NO from the solvent or gas phase. This reaction requires high NO* surface coverage, leading first to the formation of the trans-(NO)₂* intermediate with a low N−N coupling barrier (0.58 eV). Notably, trans-(NO)₂* will continue to react with NO in the solvent to form N₂O, that has not been reported. With the consumption of NO and the formation of N₂O* in the solvent, the Langmuir-Hinshelwood (L-H) mechanism will dominate at this time, and N₂O* will be reduced by hydrogenation at a low chemical barrier (0.42 eV) to form N₂. In contrast, NH₃ is completely formed by the L-H reaction, which has a higher chemical barrier (0.87 eV). Our predicted E-R reaction has not previously been reported, but it explains some existing experimental observations. In addition, we examine how catalyst activity might be improved by doping a single metal atom (M) at the NO* adsorption site to form M/Pd and show its influence on the barrier for forming the N−N bond to provide control over the product distribution.

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